A slow recovery for the star Betelgeuse –

In November 2019, the abnormal decrease in the brightness of Betelgeuse in the sky fascinated the international community with the hypothesis of its imminent disappearance. But if this final stage does not seem to have been reached yet, it has been preceded by exceptional eruptive phenomena.

Characteristics of the star Betelgeuse

Betelgeuse is one of the brightest stars in the constellation Orion (the hunter), radiating like over 100,000 suns combined. It is a red supergiant and one of the largest known stars, with a radius a thousand times that of the Sun; it could contain a billion times our star. The supergiant is so huge that if it replaced the Sun at the center of our solar system, its outer surface would extend beyond the orbit of Jupiter.

Such characteristics imply an “imminent” end for this star tumor. Betelgeuse, only a few million years old, is already nearing the end of its life, with a destiny mapped out: a supernova explosion that will make it as luminous as our entire galaxy.

Using the Hubble Space Telescope and other ground-based instruments, scientists from an international team reconstructed the scenario after and during the eruption that shook the star in late 2019.

A titanic cramp

According to scientists, the star ejected a large amount of material into space. From this eruption, a huge cloud of gas partially turned to dust as it cooled, leading to the occultation of the star.

Disturbance of the red supergiant star Betelgeuse

© ILLUSTRATION: NASA, ESA, Elizabeth Wheatley (STScI)

The star is slowly recovering from this spasm; its photosphere is gradually recovering. However, its usual pulsation cycle is disrupted: its interior now sounds like a “slightly out of tune bell”. That doesn’t necessarily mean the supergiant star will explode anytime soon, but it could continue to surprise astronomers.

In any case, the observations provide clues to how red supergiant stars lose their mass at the end of their lives, as their nuclear fusion reservoir is exhausted before exploding in supernovae. The extent of mass loss during the red supergiant phase has a major impact on their fate: in the case of Betelgeuse, it will determine whether the star will end its life as a black hole or a neutron star. The answer will be revealed to us within about 100,000 years at the latest!

For this work, several types of observational data were used. :

  • new spectroscopic and imaging data from the STELLA robotic observatory;
  • the Tillinghast Reflector Echelle (TRES) spectrograph at the Fred L. Whipple Observatory;
  • NASA Sun-Earth Relations Observatory (STEREO-A);
  • NASA’s Hubble Space Telescope, which played a significant role in understanding the phenomenon;
  • the SPHERE instrument from the European Southern Observatory (ESO) in Chile;

    and the American Association of Variable Star Observers (AAVSO).

Back to the facts

The 2019 titanic flare was likely caused by a convective plume, more than a million miles in diameter, bubbling up from deep inside the star. It produced shocks and pulsations that ejected a piece of the photosphere and left the star with a large cold surface at the site of the flare. Betelgeuse is recovering from this injury.

Equivalent to one ten-millionth of the mass of Betelgeuse, the detached piece of photosphere escaped into space and cooled to form a cloud of dust that hid the star from terrestrial observers. The loss of radiance, which started in late 2019 and lasted for a few months, was very easily visible to the naked eye.

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Also read the information
on NASA’s Hubble website [en anglais] :

Hubble Sees Red Supergiant Star Betelgeuse Slowly Recovering After Blowing Its Top

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Another striking fact: for almost 200 years, astronomers have measured the rhythm of Betelgeuse’s pulsations using variations in its brightness and its surface movements. However, these pulsations have accelerated and are currently faster than the usual 400 days. This disruption testifies to the violence of the eruption.

The star’s inner convection cells, which are responsible for its standard pulsation, can jiggle like the vibrations of an unbalanced washing machine, the researchers say. The TRES and Hubble spectra indicate that the outer layers may have returned to their normal elongation, but the surface is still bouncing around like a gelatin dessert on a plate, suggesting that the photosphere is rebuilding.

Conclusion

Directly witnessing this unprecedented event, astronomers deduced that in 2019 the star had ejected a large chunk of its visible surface. A phenomenon of such intensity had never before been detected on a star of this type. Our sun regularly experiences mass ejections from its outer atmosphere, the corona. But these events are out of proportion to what has been observed at Betelgeuse.

Therefore, particle mass ejections from red supergiants and solar coronal mass ejections are different events.

NASA’s Webb Space Telescope and ESO’s ELT may be able to detect in the infrared motion of material ejected from the star.

Reference

This research work was the subject of an article entitled “The Great Dimming of Betelgeuse: a Surface Mass Ejection (SME) and its Consequences” by Andrea K. Dupree, et.al. published in the journal “The Astrophysical Journal”.

DOI: https://doi.org/10.48550/arXiv.2208.01676

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